(a) FIELD OF THE INVENTION:
The present invention relates to an electrode for producing an electric contact on a silicon semiconductor device, and more particularly it pertains to a thin film electrode containing a thin film of a metal silicide, and still more specifically it concerns such an electrode of the type mentioned above which contains a silicide of a metal such as titanium or zirconium having a high melting point and a drastic chemically reactive property, and also it relates to a method of manufacturing such an electrode.
(b) DESCRIPTION OF THE PRIOR ART:
In the past, it is well known in the field of semiconductor devices to provide a contact hole or window at a predetermined site of a silicon semiconductor device such as IC or LSI, and to form a thin film of a metal silicide on the exposed surface portion of the silicon single or polycrystal substrate located within said window in order to make an electric contact (which usually is ohmic contact and rarely rectifying contact), by first depositing a thin film of a metal on said exposed surface by sputtering or like technique and then subjecting the resulting assembly to a heat-treatment.
Among such prior art techniques, the earliest-proposed and well known is the one developed by BTL (Bell Telephone Laboratory) which represents the formation of a thin film of platinum silicide by heat-treatment of a thin platinum film intended to provide an ohmic contact of a silicon beam lead transistor.
A brief description of the instance wherein the above-mentioned manufacturing method is applied to a silicon planar device will be made below. An Si0.sub.2 film is formed on the surface of a silicon single crystal substrate by, for example, thermal oxidation, and thereafter a contact hole or window is provided through said film by relying on the ordinary photolithographic technique. Then, a thin platinum layer is deposited by, for example, sputtering on the Si0.sub.2 film including the exposed surface of the contact hole. The resulting substrate is subjected to a heat-treatment. Whereupon, platinum reacts with silicon only within the contact hole and a platinum silicide is produced excepting the remaining Si0.sub.2 film areas which are left there intact. The resulting assembly is rinsed with aqua regia (royal water), resulting in that only platinum is selectively etched away, and a platinum silicide layer is retained in the contact hole. Thereafter, a layer of a different metal such as titanium (Ti) is deposited so as to cover the platinum silicide layer which has been already formed. Then, by an ordinary photolithographic technique, an electrode containing a platinum silicide and having a predetermined pattern is formed.
In accordance with the subsequent development of technology, there has become adopted a method of producing an electrode which is comprised of a thin film of a metal such as molybdenum (Mo) or tungsten (W) having a higher melting point to serve as a metal for providing an electric contact of a semiconductor device such as Si-LSI, or to use the silicides of these metals, i.e. MoSi.sub.2 or WSi.sub.2, for the production of an electrode to make electric contact with the silicon surface of the device by subjecting these metals in their state of contacting the silicon surface of the substrate to a heat-treatment. These electrodes and their manufacturing methods have been accepted as important techniques for the formation of contact electrodes of semiconductor devices such as said LSI which requires a high degree of precision or high packing density, or for the formation of micro-lead films.
The above-described electrode which contains the formation of a platinum silicide by the use of platinum is relatively easy to manufacture since it requires only one step of photolithography. However, in view of the platinum being a very expensive metal as one reason, there have become employed molybdenum and tungsten in place of the precious platinum However, the attempt to further develop the above-mentioned technique by using a metal film of Ti or Zr having a higher reactivity and a higher melting point for the formation of a silicide of these metals through a heat-treatment, however, has been found to be entailed by various problems
That is, as an example of such problems, description will be made with respect to an instance wherein an electrode containing a titanium silicide is formed by using titanium in place of platinum. During a heat-treatment which is conducted in the state of the titanium film covering not only the exposed silicon surface portion located within the contact hole but also being stacked over the Si0.sub.2 film, a problem was found to arise that a titanium silicide was formed not only within the contact hole but also on the Si0.sub.2 film as well. This formation of titanium silicide on the Si0.sub.2 film is due to the following reasons that titanium has chemically more active characteristics for oxygen atoms rather than for Si atoms, so that this reaction breaks the bonding between silicon atoms and oxygen atoms in the silicon oxide, and the resulting freed silicon atoms react with titanium atoms.
A titanium silicide layer is thus formed not only within the contact hole alone but also on the entire surface of the Si0.sub.2 film as well. It is, therefore, impossible to selectively and discriminatingly etch only that Ti silicide film portions located on the Si0.sub.2 film, unlike the instance of the platinum film wherein such a discriminating etching is possible Hence the need to conceive a specific counter-measure therefor. As one example of the counter measure, there is the technique that a layer of titanium film is formed first on the entire surface, and then by relying on the photolithography, the titanium layer is retained only in the area substantially corresponding to the contact hole, and thereafter a titanium silicide layer is formed in the region substantially corresponding to the contact hole. In case such a method is adopted, there will occur such inconveniences that oxygen atoms are taken in that portion of titanium silicide which is in contact with Si, or that the surface of titanium silicide is oxidized, degrading its quality as an electric contact with the metal film which is stacked thereon, or that there appears variance of value of the contact resistance, or that the otherwise avoidable surplus photolithographic step becomes necessary.